The present invention relates to a method for protecting an aircraft in cruising flight against excessive load factors when vertical gusts of wind occur, and to a device for the pitch-attitude control of an aircraft employing this method.
The vertical gusts of wind to which the present invention relates are swirling turbulence (rather than high-frequency turbulence which causes vibration), generated by swirling rollers of the vortex type, which subject the aircraft passing through or close to them to an upward (or downward) gust of wind and then to a downward (or upward) gust of wind for a few seconds, or alternatively to a succession of upward and downward gusts of wind, if there are a number of vortices.
It is known that commercial aircraft comprise a pitch-attitude flight control system which is under the control of the pilot, an automatic pilot, switching means allowing the aerodynamic pitch-attitude control surfaces (flaps, trim) of said aircraft to be controlled either by said controlled flight control system or by said automatic pilot, and protection means, especially as regards incidence, capable of acting on said switching means to disconnect said automatic pilot and switch on said controlled flight control system, when the angle of incidence, as it increases, reaches an excessively high value.
If the aircraft, in cruising flight with the automatic pilot in operation, encounters an upward gust of wind of this kind, it is firstly subjected to a high positive load factor. The automatic pilot reacts to the variations in longitudinal pitch attitude, but its authority is limited, which means that its reaction is weak and it becomes disconnected by the action of said protection means and of said switching means as soon as the angle of incidence reaches a protection threshold. Control is then by means of the controlled flight control system which generates a strong order to nose down in order to reduce the angle of incidence and counter the effect of the upward wind. However, the gust then changes direction and the wind becomes a downward gust, which means that now the pilot's nose-down action is added to the downward gust of wind and applies a negative load factor on the aircraft, this load factor being even higher than the factor that would result from the gust itself. This may result in injury to passengers and damage to the aircraft.
A similar process occurs if the aircraft was being flown manually when said gust occurred.
Of course, what has been described hereinabove applies mutatis mutandis to the case where a downward then upward gust of wind is encountered.
Thus, from the foregoing, it will be readily understood that in general, any control action to counter a gust of wind produces its effect at the moment said gust changes direction, which means that this control action unfavorably increases the already high load factor to which the said gust is subjecting the aircraft.